The present invention relates to an image carrier for use in image forming apparatus such as electrophotography or facsimile, on which a latent image is to be formed, and particularly relates to an image carrier having a photoconductive layer for forming a latent image by optical writing, and having a large number of conductive particles to be charged due to charge injection.
Japanese Patent Publication No. 6-3921A discloses an image forming apparatus in which an image carrier is charged by charge injection so that an electrostatic latent image is formed on the image carrier. In this example, a photoconductive drum serves as the image carrier. The photoconductive drum has a photoconductive layer and a charge injection layer. The photoconductive layer is comprised of a charge generation layer for generating charges in response to received light, and a charge transport layer for transporting the generated charges to the surface of the photoconductive drum. The charge injection layer is formed to have a predetermined film thickness on the charge transport layer in such a manner that conductive filler made of SnO2 in the form of a large number of conductive particles is dispersed into binder resin made from phosphagen resin.
A charger is then brought into contact with the charge injection layer so as to inject charges into SnO2 of the charge injection layer, so that the photoconductive drum is charged uniformly. Next, the photoconductive drum is exposed to a laser beam. The charge generation layer receiving the laser beam generates charges, and positive charges of the generated charges are transported to the surface of the photoconductive drum through the charge transport layer. Thus, the charges are eliminated from the exposed portion of the photoconductive drum surface so that a latent image is formed thereon.
Japanese Patent Publication No. 9-218566A discloses an image carrier having a similar configuration.
When the photoconductive drum is charged in such a charge injection system, charging can be achieved at a low voltage. Accordingly, the power supply cost can be reduced, and generation of ozone products can be prevented. Further, when there is a pin hole or the like in the photoconductive drum, there may occur a failure in image formation due to leakage from the pin hole or the like. Due to charging at a low voltage, however, such a failure can be suppressed to the utmost.
Assume that image formation is performed over a long time period in the above photoconductive drum, the surface layer of the photoconductive drum will be worn by members abutting against the photoconductive drum. Since the above photoconductive drum is arranged to have a charge injection layer only in the surface layer of the photoconductive drum, the charge injection layer will be worn down, thereby charging by the charge injection cannot be performed, and image formation cannot be performed. In other words, the film thickness of the charge injection layer which is the surface layer of the photoconductive drum determines the life of the image forming apparatus. It is therefore difficult to perform charging the photoconductive drum stably over a long time period.
When the surface of the photoconductive drum is filmed by coating of a resin component of toner or the like due to long-term use of the photoconductive drum or the like, charges cannot be injected into the charge injection layer reliably. It is therefore necessary to perform image formation while grinding the surface layer of the image carrier, that is, the charge injection layer positively to thereby remove the filming. However, when the charge injection layer is ground, the film thickness of the charge injection layer determines the life of the image forming apparatus in the same manner as in the aforementioned case where the charge injection layer is worn away. It is therefore difficult to perform charging the photoconductive drum stably over a long time period.
The present invention relates to a developing device for use in the above image forming apparatus to develop a latent image formed on an image carrier, and particularly relates to a FEED type developing device using toner having a minute particle size.
The present invention also relates to image forming apparatus incorporating such an image carrier and such a developing device.
Japanese Patent Publication No. 6-202465A discloses a FEED type developing device comprising a developing roller a surface of which in provided with both of minute conductive regions and minute dielectric regions. A functional layer mainly comprised of carbon is formed on the dielectric regions. In this device, nonmagnetic single-component toner, to which external additive is added if necessary, is supplied onto the developing roller. The toner is held on the developing roller by minute closed electric fields formed in the vicinity of the surface of the developing roller, and carried onto an image carrier on which an electrostatic latent image which has been formed to make the latent image visible as a toner image.
FIGS. 20 to 22 show a case where such a developing device of the FEED type is combined with an image carrier of the charge injection type.
As shown in FIG. 20, an image forming apparatus 21 comprises an image carrier 22 for forming a latent image thereon and for carrying a toner image, a charger 23 having a charging roller 23a for charging the image carrier 22 uniformly, an exposer 24 for writing an electrostatic latent image on the image carrier 22 by exposure, a FEED type developing device 25 having a developing roller 25a serving as a developing roller, a control blade 25b for regulating a thickness of toner on the developing roller 25a, and a developing bias power supply 25c for supplying a developing bias voltage to the developing roller 25a, and a transferer 26 having a transfer roller 26a. 
The image carrier 22 is comprised of a substrate 22a made from a conductive material such as aluminum, an under coated layer 22b formed on the substrate 22a, a photosensitive layer 22c formed on the under coated layer 22b, and a charge injection layer 22d formed on the photosensitive layer 22c. The charge injection layer 22d has a configuration in which a large number of conductive particulates (not shown) have been independently dispersed in binder resin 27.
As shown in FIG. 21, the developing roller 25a of the FEED type developing device 25 is comprised of a core member 25d made from a conductive member, and an insulative portion 25e formed on the core member 25d. A large number of independent floating electrodes 25f are provided in the surface layer of the insulative portion 25e. The independent floating electrodes 25f are exposed on an outer circumferential surface 25e1 of the insulative portion 25e while the tip end faces thereof are made flush with the outer circumferential surface 25e1. In addition, the developing device 25 has a toner supply roller 25g for supplying toner to the developing roller 25a. 
More specifically, toner T stored in a toner container of the developing device 25 is supplied to the toner supply roller 25g. As shown in FIGS. 21 and 22, the toner supply roller 25g rotates in the same direction as the developing roller 25a so as to charge the developing roller 25a and the toner T, so that the toner T is attached onto the developing roller 25a. When the developing roller 25a rotates further, the control blade 25b stabilizes the charged condition of the toner T while regulating the thickness of the toner T on the developing roller 25a. Thus, the adhering toner T reaches a development point D where the latent image on the image carrier 22 is to be developed.
In the development point D, the latent image is developed with the toner T in a contact or non-contact manner. Here, if necessary, a developing bias voltage and a supply bias voltage (DC, AC, DC superimposed AC, pulses, etc.) are applied from the developing bias power supply 25c and a supply bias power supply 25h to the developing roller 25a and the toner supply roller 25g respectively. Thus, the developed image can be optimized.
Next, description will be made on a mechanism of toner adhesion to the FEED type developing roller 25a. The developing roller 25a is designed so that a large number of minute independent floating electrodes 25f are independently dispersed in the surface layer of the insulative portion 25e, as shown in FIG. 22.
The toner T is attached as follows. First, a portion of the developing roller 25a which has been subjected to the development comes into contact with the toner supply roller 25g. Residual toner T′ staying on the developing roller 25a is scraped mechanically and electrically by the toner supply roller 25g, and received in the toner tank in the developing device, while the independent floating electrodes 25f are charged by friction. Due to this frictional charge, charges of the developing roller 25a and the residual toner are unified (initialized). Next, the toner T conveyed from the toner tank by the toner supply roller 25g is charged by friction so as to electrostatically adhere to the independent floating electrodes 25f of the developing roller 25a. 
As for polarities at this time, the toner T has an inversed polarity to the charges on the image carrier, while the independent floating electrodes 25f of the developing roller 25a have the same polarity as the charges on the image carrier. Incidentally, as shown in FIG. 22, minute closed electric fields E each having a large gradient are formed, thereby the toner T can be attached thereto in multiple layers. Further, due to the closed electric fields E, the toner T adhering to the independent floating electrodes 25f is hardly separated from the developing roller 25a. The toner T is carried to the development point D while its thickness is regulated by the control blade 25b. At the development point D, the electric field condition is made such that the toner T on the developing roller 25a is readily adhered onto the image carrier 22.
Then, the charging roller 23a of the charger 23 is brought into contact with the charge injection layer 22d so as to inject charges into the conductive particulates of the charge injection layer 22d of the image carrier 22 due to the charging voltage of the charging roller 23a. Thus, the image carrier 22 is charged uniformly. When the charged image carrier 22 is exposed to light by the exposer 24, a latent image is written on the image carrier 22. In the developing device 25, the toner supply roller 25g to which the supply bias voltage is applied conveys the toner T onto the developing roller 25a. The developing roller 25a to which the developing bias voltage is applied carries the toner T onto the image carrier 22.
The latent image written on the image carrier 22 is developed with the toner T conveyed by the developing roller 25a. The toner image TI obtained thus is transferred to a not-shown recording medium such as paper by the transfer roller 26a of the transferer 26. The toner image transferred to the recording medium is fixed by a not-shown fuser. Thus, an image is formed on the recording medium.
Assume that such a single-component minute toner is used in a non-FEED type developing device. A developing roller of such a developing device has mechanical surface roughness. Therefore, the developing roller is filmed with the minute toner when the minute toner is conveyed by the developing roller and regulated by the control blade. This is because smaller toner particles of the minute toner intrude into the minute irregularities of the developing roller surface so that the irregularities are filmed with the smaller toner particles.
Therefore, when the FEED type developing device shown in FIGS. 20 and 21 is used, the toner T electrostatically adheres to the independent floating electrodes 25f, and the toner T adhering to the independent floating electrodes 25f is hardly separated therefrom. Thus, filming of the developing roller 25a with the toner T is suppressed.
However, in the above described FEED type developing device, the independent floating electrodes 25f are flush with the outer circumferential surface 25e1 of the insulative portion 25e. Therefore, as shown in FIG. 22, the toner T also adheres to the insulative portion 25e in the vicinity of the independent floating electrodes 25f. When printing is performed over a long time period, the circumferential edge portions of the independent floating electrodes 25f and the insulative portion 25e in the vicinity of the independent floating electrodes 25f are filmed with the toner adhering to the insulative portion 25e. As a result, the area where the toner T to be used for development can occupy the independent floating electrodes 25f is made small, so that not only the toner carrying capacity is changed but also a required toner carrying ability cannot be secured. Thus, there occurs a failure in image formation.